Monkeypox is a rare disease that does not spread easily between people. It’s considered mild and typically occurs in remote parts of central and west Africa, according to the World Health Organization. The virus lives in animals but can be transmitted to humans. In 2017 Nigeria experienced its first outbreak of the viral illness since 1978, according to WHO.

“It is likely that monkeypox continues to circulate in Nigeria and could, therefore, affect travelers who are returning from this part of the world, however, it is very unusual to see two cases in such a relatively short space of time,” Dr. Nick Phin, deputy director of Public Health England’s National Infection Service, said, according to the Independent.

The virus is less deadly than smallpox, with symptoms appearing about two weeks after exposure. Patients may experience fever, headache, loss of appetite, muscle aches and swollen lymph nodes, according to Live Science.